The antiviral activity of 6–0-butanoylcastanospermine (MDL 28,574) [50% inhibitory concentration (IC50:1.1 μM)] in JM cells infected with a recent isolate of HIV-1 (GB8), was compared with other inhibitors of glycoprotein-processing enzymes. N-butyldeoxynojirimycin (BuDNJ), deoxynojirimycin (DNJ), castanospermine (CAST) or the reverse transcriptase inhibitor 2′3′-dideoxycytidine (ddC) had activities of 56, 560, 29 and 0.1 μM, respectively. MDL 28,574 was at least 50 times more active than BuDNJ and less active but better tolerated in cell culture than ddC, two compounds currently undergoing clinical trials. The CAST derivative showed good protection in H9 cells infected with HIV-1 (RF; IIIB; U455), and HIV-2 (ROD), although the potency was less than that seen in the JM/GB8 system. HIV-1 glycoproteins, gp160 and gp120, synthesized in H9 cells chronically infected with HIV-1 (RF) and treated with MDL 28,574, were characterized by an increase in relative molecular weight of approximately 7–8000 kD. The ratio of gp120 to gp160 was markedly reduced in treated cells and provided further evidence that cleavage of the gp160 precursor molecule is a major consequence of the inhibition of glycoprotein processing. The intracellular target for MDL 28,574 was verified as α-glucosidase-l of the processing enzymes by the analysis of high-glucose glycopeptides recovered from treated mouse cells. This activity correlated with the antiviral effect observed against the growth of a mouse retrovirus, Moloney murine leukemia virus (MOLV), in mouse cells.